COVID-19 Science Table Guidance on Antivirals

The summary guidance on all treatment medications for COVID relevant to primary care is in the infographic at the end of this section as well as a detailed section about evidence and important prescribing safety information for Paxlovid. 


Paxlovid access for eligible patients is now primarily through primary care. For those patients who have complex interactions / are potentially ineligible for Paxlovid, remdesivir is accessed only through the Hamilton COVID Care Clinic (CCC) and outside Hamilton through other CACs (see Hamilton CCC Referral form lower down page). Sotrovimab is no longer recommended as effectiveness has changed against newer variants, so for those unable to take Paxlovid 3 days of Remdesivir will be offered. See below for list of pharmacies offering access to supply.

Criteria for Use

WHO (Eligibility): Antiviral (Remdesivir/Paxlovid) are now available for consideration for mildly ill patients who present within 7 days of symptom onset (5 days for Paxlovid) and meet any one of the following eligibility criteria for being in a high risk population.

As of April 11, 2022, the following higher-risk groups are eligible to be tested and assessed for antiviral treatments, such as Paxlovid, in Ontario.

  • Individuals aged 18 and over who are immunocompromised (have an immune system that is weakened by a health condition or medications);
  • Individuals aged 70 and over;
  • Individuals aged 60 and over with fewer than three vaccine doses; and
  • Individuals aged 18 and over with fewer than three vaccine doses and at least one risk condition (e.g., a chronic medical condition)

Risk conditions include: diabetes, obesity, heart disease, hypertension, congestive heart failure, chronic respiratory disease (including cystic fibrosis), moderate or severe kidney disease, intellectual or developmental disability, cerebral palsy, sickle cell disease, moderate or severe liver disease.

The “eligible” population is wider than that in the trial population so any likely benefit to the individual patient should be considered in the light of the tip sheets (see helpful tools) and evidence summary below.

Although pregnancy is a risk factor for severe COVID-19, the absolute risk for this population remains low due to the young age and lack of comorbidities of most pregnant individuals. There is no data confirming safety of either Paxlovid or remdesivir in pregnancy. Considerations for the use of specific COVID-19 therapeutics in unvaccinated pregnant women should therefore be made on a case-by-case basis.

Breastfeeding: Warn breastfeeding patients that the effects of Paxlovid on the breastfed infant are not known and they should therefore avoid breastfeeding during and for the 7 days following treatment.

Helpful quick tools

Paxlovid Antiviral

Information updated March 2, 2022

What is it?

Paxlovid consists of a drug that blocks an enzyme the coronavirus needs to reproduce – a similar mechanism to some anti-HIV drugs – as well as another drug that slows the drug’s breakdown. It has been approved for use in Canada in Jan 2022.

The only data available on the medication is from a single trial in an unvaccinated population (pre-omicron). This RCT among 2,246 high-risk non-hospitalized patients who were unvaccinated was reported to reduce the risk of hospitalization.

Over 28 days 0.7% vs 6.5% were hospitalized in paxlovid vs placebo arms (p<0.0001). This represents an approximate number needed to treat (NNT) of 17 in high-risk unvaccinated patients with COVID delta variant to prevent 1 hospitalization (Deaths were 0% vs 1.3%). NNT 77 for the same subgroup. Laboratory based data suggests an effect is likely to hold for the omicron variant.

The only evidence is for prevention of hospitalisation in high risk patients.

There is no evidence for effect on symptoms or post COVID symptoms. Adverse effects were the same in both arms.

MOST COMMON SIDE EFFECTSOF PAXLOVID: dysgeusia (making everything taste bitter, sour or sweet), diarrhea, hypertension, headache and myalgia.

Pregnant or breastfeeding women, those with active HIV (viral load > 400 copies/ml) or on particular HIV medications, those with liver disease and significant renal impairment and low risk fully vaccinated individuals were excluded from the trial. You can read the trial publication here.

DOSE: 3 tablets (150 mg nirmatrelvir 2 tablets; 100 mg ritonavir 1 tablet) taken together twice daily for 5 days. It is important that all tablets are taken together twice daily. (note there is a dose adjustment for renal impairment which is why the nirmatrelvir comes as 2x150mg)

UNCERTAINTIES: there are no trial results of effectiveness in vaccinated individuals, or with omicron variants. Due to the reduced risk of hospitalization in both of these contexts it is likely that the NNT will be higher where the risk of hospitalization is lower (e.g. vaccinated population and milder variant). For example if the risk of hospitalization is 0.9% the NNT to prevent hospitalization would be 125 (this was the estimated risk of hospitalization in Ontario in the Delta wave in 2021 for the non-Long Term Care population). This NNT will therefore likely be even higher for the Omicron variant.  For this reason treatment is focused on those at higher risk where the potential benefit is clinically significant.

Other important prescribing information

There is quite a long list of contraindications as well as potential serious interactions that require changes in drug dosing or scheduling. The quick interaction checker tools in the “helpful quick tools” section above as well as the product monograph offer good guidance and asking a pharmacist to review where possible as a double check is also recommended.

1. There is a contraindication / dose adjustment for impaired renal function

Contraindicated if GFR <30ml/min; If GFR 30-60 dose adjust to:

150 mg of nirmatrelvir (one tablet rather than two) along with 100 mg of ritonavir, taken together, twice daily, in the morning and evening

2. Paxlovid is not recommended for use in severe hepatic dysfunction

3. There is no data on use in pregnancy and breastfeeding so there is an “only if the risks outweigh the benefits” statement– see full product info sheet for full information.

4. Paxlovid is CONTRAINDICATED with drugs that are highly dependent on CYP3A for clearance. There are also a large number of potentially serious interactions that REQUIRE DOSE CHANGE OR SCHEDULE MODIFICATION. See Table 1 for contraindicated medications and Table 4 for serious interactions requiring modification of therapy or monitoring in the attached tables from the drug datasheet.  Commonly used medications in primary care are flagged in green.  

5.  In addition Paxlovid cannot be started immediately after discontinuation of any of the following medications due to the delayed offset of the recently discontinued CYP3A inducer

Anticancer drugs: apalutamide

Anticonvulsants: carbamazepine, phenobarbital, phenytoin

Antimycobacterials: rifampin

Herbal products: St. John’s Wort (hypericum perforatum)

Health Canada approved Product Monograph including patient medication information

Writing a prescription for Paxlovid

  • When writing a prescription, clearly indicate the following:
    • Confirmed COVID-19
    • Symptomatic for 5 days or less (symptom onset is considered day 0)
    • Increased risk for COVID-19 infection progression [indicate to see above list]
    • eGFR (if known)
    • Send up to date current medication list so pharmacist can do a final interactions check and if appropriate consider adding a note on the prescription “feel free to contact re: drug interactions or concerns”.
    • Include information for pharmacist about decisions on adjustments to other medications
  • Consider renal function and dose adjust accordingly for safety:
    • Normal renal function (eGFR > 60mL/min): 300 mg nirmatrelvir (two 150 mg tablets) with 100 mg ritonavir (one 100 mg tablet), with all three taken together orally twice daily for 5 days
    • Mild-moderate renal impairment (eGFR 30-60 mL/min): 150 mg nirmatrelvir (one 150 mg tablet) with 100 mg ritonavir (one 100 mg tablet), taken together orally twice daily for 5 days.
    • Nirmatrelvir/ritonavir is not recommended for patients with eGFR < 30 mL/min, or severe hepatic impairment (Child-Pugh Class C)
  • Inform patient of the pill burden (6 tablets daily) and emphasize the importance of adherence once initiated to minimize the development of drug resistance.
  • Caution patients to avoid any contraindicated medications used intermittently, e.g. agents for erectile dysfunction, colchicine, loratadine.
  • Advise patients at risk of conceiving to use contraception during and for the 7 days following treatment.
  • Advise patients using oral contraceptives containing ethinylestradiol to use additional contraceptive precautions.
  • Inform the patient of common side effects: dysgeusia (making everything taste bitter, sour or sweet), diarrhea, hypertension, headache and myalgia.
  • Monitor as per usual for clinical illness in high risk patients, and additionally if indicated by potential drug interactions

Where to get Paxlovid (Pharmacies):

Ontario Health have indicated that list of pharmacies that are dispensing Paxlovid will be available at as of Wednesday, April 13 at 8:00 a.m. The list will be regularly updated as the list of participating pharmacies expands.

Referral Process to CCC:


Note: the CCC will still take some referrals for complex patients with significant interactions who may require remdesivir.

Referral Form (NOTE: please send an up-to-date complete medication list with all referrals, completed by a pharmacist where possible)
*Instructions for Patients

*April 18: The Telus PS forms will be updated as available

Outside Hamilton

Different local pathways will apply. Paxlovid will be accessed through primary care and also referral to your local COVID Care Assessment Centre where remdesivir will also be available for those eligible (see your local CAC for referral forms and process).

COVID-19 testing locations and clinical assessment centres
COVID-19 antiviral treatment in Ontario

Leave a Reply